Emergency vehicles such as police cars, ambulances and fire trucks are required to perform under a wide range of driving conditions and at a wide range of speeds. For example, during the time that a police car is in pursuit of a suspect car, the police car may be driven at a high rate of speed for a given time, at a reduced speed while crossing critical intersections, and stopped once the vehicle is halted.
Under these varying conditions, the primary responsibility of the emergency signalling system is to warn the public of the presence of the vehicle, since the emergency vehicle is often driven in a non-standard manner during such extraordinary times. To effectively warn the public, conventional lights and sirens provide recognizable patterns and various colors or tones.
In essence, the flashing or strobing lights of the vehicle are broadcasting information to potential observers such that once detected, the observer is warned of the abnormal driving conditions that may soon be encountered so that appropriate precautionary measures may be taken. For example, light bars, composed of anywhere from two to twelve light elements, wherein each element is ordinarily capable of flashing at a rate of between 60 and 240 flashes per minute (one to four hertz), may be attached to vehicles to broadcast the warning information, as standardized by the Society of Automotive Engineers (SAE J-845, SAE J-595, SAE J-1318) for emergency warning lights. The lights may be filtered so that non-white colors allow the observer to differentiate between the white and red lights of ordinary vehicles and also to differentiate between the types of emergency vehicles. In short, the combination of flashing or strobing lights, at a particular pattern and of a distinguishable color signifies to the public the presence of an active emergency vehicle.
In general, a warning issued by an emergency signalling device attempts to maximize the effectiveness of the warning by including enough information to overcome observational difficulties that are potentially present in the general public. For example, emergency sirens often sweep through a range of frequencies at varying rates. The maximum and minimum frequencies are arranged so as to be within the most efficient detection portion of the human audio spectrum. The purpose of sweeping through a range of frequencies is to ensure that potential human observers will hear the siren over background noises and because many potential observers suffer from hearing loss at certain frequencies.
The rate of sweeping between the frequencies is also chosen to impart information, usually indicative of urgency, to the public. For example, a frequency pattern known as a "Wail" sweeps from approximately a 500 hertz tone to a 1500 hertz tone in ten to thirty sweeps per minute. A frequency pattern known as a "Yelp" sweeps between the same audible tones but at a much faster rate of approximately 150 to 250 sweeps per minute. Thus, the "Yelp" pattern is able to rapidly impart the signalling information, even to persons having limited ability to hear certain frequencies, so that a faster reaction than with the "Wail" tone is possible.
Parallel concepts are available in emergency lighting devices as well. For example, light bars provide a variety of colors in a single light package. These varying colors aid the stimulation of unoccupied visual channels in human observers and overcome visual deficiencies such as color blindness present in some potential observers. Additionally, having a greater number of flashing light elements provides a greater number of flashes to the eye, which potentially results in faster reaction times.
However, even when such flashes are noticed, the speed of the vehicle is somewhat uncertain until the vehicle itself is visually detected from a relatively close range. To some extent, with audible sirens an increase or decrease in volume in conjunction with the frequency shift resulting from the Doppler effect provide some hint of vehicle's relative speed and direction (i.e., velocity) to most observers, however there is no consistent pattern provided thereby that such observers can ordinarily recognize in any significant time. Moreover, non-stationary observers influence these effects with their own velocity, making it even more difficult to deduce an emergency vehicle's velocity from its siren when in motion relative to it. This is because the frequency shift and amplitude change over time will be different to a stationary observer, to an observer moving toward the vehicle and to an observer moving away from the vehicle.